The present invention relates to an image signal
processor for reading and printing a character or a pattern drawn on a white board, a blackboard, or the like.
There exists an image pickup and printing apparatus which is used in a manner such that a character, a pattern, or the like which is drawn on a screen, such as a white board, is projected through a lens while being monitored through a viewfinder which determines the angle of view for the projected object. An image of the projected object is created by an image pickup element, such as a charge-coupled device, which generates an image signal to operate a printer for printing the image on paper. That apparatus is mainly used to read a character, a pattern, or the like drawn in a dark color, such as black or red, on a light-color background, such as a white board. That apparatus cannot be directly used to read a character, a pattern, or the like drawn in a light color, such as white or yellow, on a dark-colored background, such as a blackboard.
To accomplish the reading operations in the existing apparatus, the background and the characters patterns are converted into mutually-reversed binary signals with respect to an image signal supplied from the image pickup element. The image signal is usually subjected to automatic gain control to stabilize an output signal. For that reason, when that apparatus is used to read the characters or the like drawn in a light color on a dark-colored background, the potential level of an output signal component for the dark-colored background is too high to perform binary encoding, as shown in FIG. 4.
When the existing apparatus is used to read the characters or the like drawn in a dark color on a light-colored background, the potential level of an image signal component for the characters or the like is lower than the potential level of an image signal component for the background. The image signal component for the background is, therefore, subjected to the automatic gain control so as to set the potential level of an output signal component near the saturation level of an amplifier, thereby making the amplitude of an output signal component for the character or the like large.
On the other hand, when the existing apparatus is used to read the characters or the like drawn in a light color on a dark-colored background, an image signal component for the background is subjected to the automatic gain control to set the potential level of an output signal component for the background near the saturation level of the amplifier. In that case, however, since the potential level of an image signal component for the character or the like is higher than the potential level of the image signal component for the background, the amplitude of an output signal component for the character or the like cannot be made large, as shown in FIG. 4.
For that reason, it is difficult to compare the potential level of the output signal component for the character or the like with the potential level of a comparison signal to perform binary encoding. Therefore, although the existing apparatus can be used to read a dark-colored character or the like drawn on a light-colored background, that apparatus cannot be directly used to read a light-color character or the like drawn on the dark-colored background, thereby establishing a problem with that apparatus.